Non-volatile memory devices are used in certain applications where data must be retained when power is disconnected. Applications include general memory cards, consumer electronics (e.g., digital camera memory), automotive (e.g., electronic odometers), and industrial applications (e.g., electronic valve parameter storage). The non-volatile memories may use phase-change memory materials, i.e., materials that can be switched between a generally amorphous and a generally crystalline state, for electronic memory applications. The memory of such devices typically comprises an array of memory elements, each element defining a discrete memory location and having a volume of phase-change memory material associated with it. The structure of each memory element typically comprises a phase-change material, one or more electrodes, and one or more insulators.
One type of memory element originally developed by Energy Conversion Devices, Inc. utilizes a phase-change material that can be, in one application, switched between a structural state of generally amorphous and generally crystalline local order or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. These different structured states have different values of resistivity and therefore, each state can be determined by electrical sensing. Typical materials suitable for such applications include those utilizing various chalcogenide materials. Unlike certain known devices, these electrical memory devices typically do not use field-effect transistor devices as the memory storage element. Rather, they comprise in the electrical context, a monolithic body of thin film chalcogenide material. As a result, very little area is required to store a bit of information, thereby providing for inherently high-density memory chips.
The state change materials are also non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until reprogrammed, as that value represents a physical state of the material (e.g., crystalline or amorphous). Further, reprogramming requires energy to be provided and dissipated in the device. Thus, phase-change memory materials represent a significant improvement in non-volatile memory technology.
In an effort to improve scaling of the memory device to improve the density of memory arrays, one technique is to reduce the area of a bottom contact. However, focusing on only the bottom contact area leads to asymmetrical top and bottom contact areas. That is to say, the bottom contact is small and the upper contact is large in comparison with each other. Thus, a “mushroom”, which is the volume of phase-change material that changes state when the memory device is programmed, is small near the bottom contact and large near the top contact. A “mushroom” shape is described with respect to FIG. 15 of U.S. Pat. No. 7,005,666 (issued Feb. 28, 2006), the contents of which is incorporated herein by reference in its entirety. Moreover, the efforts in reducing the size of the bottom contact create highly non-symmetrical contact areas. The result is that the programming current and offset current are large due to the volume of phase-change material that must be switched. These memory devices do not reduce the amount of programming current required to operate the memory device because, even with a low area bottom contact, the volume of phase-change memory being switched is large. This leads to an “offset current” where when plotting programming current vs. bottom electrode contact area, a positive offset current is observed when extrapolating to zero area.
Other attempts to reduce the offset current include a ring-type top contact. However, the switched phase-change volume and the offset current are not dramatically reduced. Yet another method includes increasing the resistivity of the bottom contact and modifying the shape of the bottom contact. In some cases, the programming current may be reduced but the offset current is not dramatically reduced. Yet another method uses a resistive layer (e.g., a breakdown layer) between the top and bottom contact that is broken down in a small area in the first switching operation. However, consistency of operation may be difficult to control in mass production.
Therefore, a need has arisen to reduce reset and offset current of a phase-change memory device. Moreover, it is desirable to reduce the manufacturing costs and process difficulties for producing the bottom contact. Additionally, it is desirable to increase the stability and cycle-life of the memory devices through reduced programming currents.